If several data stations use a single transmission channel in common, access to this channel must be so organized that at any time only one station can send data, e.g., in packet form. This may be accomplished by a pure time division multiplex operation, in which dedicated time slots of a cyclic time frame are allocated for a long time interval, or, on the other hand, the assignment of temporary short time slots upon request.
For this latter mode of operation several methods are known in the art. In polling systems a central location polls the data stations sequentially. If a data station has data ready to send, it provides an appropriate response, and thereupon the channel is allocated to it temporarily. One disadvantage of this type of operation is that all data stations, including those which are not ready to send, are polled. This results in a relatively high percentage of unused channel capacity. An additional disadvantage is the need for a sophisticated central control station.
Other known systems use the principle of random access. In this type of operation, each data station can send a data packet when it is ready, a synchronized partitioning of the channel time into time slots being provided. In such systems no central control is required. These systems, however, suffer from data collisions since data packets are frequently sent during the same time slot from different data stations and must then be repeated at a later time. This also increases the undesired percentage of unused channel capacity.
Furthermore, loop structured transmission systems are known in the art in which prior to or at the beginning of a time frame a request field circulates, into which each data station which is ready to send can enter its address or priority class. When this field returns to the control unit, it contains the highest address or class which requested channel allocation (in a first known system), or a superimposed representation of all classes requesting channel allocation from which it must select one (in another known system). A disadvantage is that in each frame channel allocation is effected to one data station or one class only while the others must repeat their request, possibly several times. Furthermore, these methods are restricted to loop structured transmission systems which in some environments is unreliable.